Freshwater Bryozoa, with descriptions of new Species. 175 



exserted, there is a delicate membranous cup, PI. IV. fig. 1 d, 

 projecting upwards from the inner surface of the mouth of the 

 cell. This cup is the homologue of the circle of setae surrounding 

 the aperture of Bowerbankia and other marine genera. In Palu- 

 dicella the tunic is sprinkled with large nucleated cells, fig. 4 m, 

 and at certain intervals bends abruptly inwards, figs. 1 & 2 u, r, 

 dividing the polypidom into cells at the points indicated by the 

 constrictions in the horny tube. Thus each polype is isolated, is 

 contained in fact within a distinct membranous cell, the end-walls 

 of which abut against the end-walls of the adjoining cells. The 

 divisions are therefore double, and being of living membrane and 

 in contact, it is probable that all the inhabitants of the polypidom 

 are in some degree connected in vital action. The end-walls are 

 considerably thickened in the centre, forming a bulb or boss pro- 

 jecting into the cell. The polypes of Fredericella are not sepa- 

 rated the one from the other, though a few divisions appear to 

 exist at distant points. Thus it would seem that groups of ani- 

 mals are associated together as it were in one tube. Neither in 

 Plumatella are the polypes separated. 



The polype lies in the longitudinal axis of the cell, Plates II. 

 & IV. figs. 2, 2, being provided with numerous muscles for pro- 

 trusion and retraction. It is held in its place principally by a 

 membranous tube — the tentacular sheath, PI. II. fig. 2 m, n, and 

 PI. IV. fig. 2 d', d l y which blends with the inverted lips of the 

 tunic, PI. II. fig. 2 /, a little below the orifice of the cell, and con- 

 tinuing downwards within the cell incloses the bundle of re- 

 tracted tentacles, and is attached round the tentacular disc a'. 



Digestive System. — The organs of digestion, comprising nearly 

 the whole of the polype, float freely in the visceral cavity. The 

 entrance to the alimentary canal is furnished with tentacles, 

 PI. II. fig. 1 b ; these arise from a margin surrounding the oral 

 opening in two different fashions ; in the one they form a com- 

 plete circle round the mouth, in the other they are arranged in 

 a crescentic manner, the limbs of the crescent being two arms, 

 PI. III. figs. 1, 2 & 3 c, e & c, extending from the sides of the 

 mouth, fig. 3 a, having their bases confluent and with a row of 

 tentacles on their inner and outer margins. Paludicella and 

 Fredericella, PL IV. fig. 1/, and PL II. fig. 1 b, are examples of 

 the first mode of arrangement; and Plumatella, PL III. figs. 1, 2 

 & 3, and Alcyonella of the second. In Paludicella the tentacles 

 when spread out form a very exact inverted cone, closely resem- 

 bling the shape they assume in some of the marine species. The 

 base or disc supporting the tentacles is not exactly circular in 

 Fredericella ; in this genus it is a little flattened at the point cor- 

 responding to the space between the oral arms in Plumatella ; 

 and there is also a delicate transparent membrane, PL II. fig. 1 



